if I would put two of this in series, calibrating the cutoffs and using a stereo pot for resonances, will I have 24db/oct or will the cutoff cvs respond
too differently? should I build 200 and then select a matched pair? :DDD

I've got a version of this circuit running with a couple of 0.01uF caps and a 220k resistor across the op-amp. The cutoff works beautifully, but adjusting the resonance knob only alters the sound for the last 1/8th of the knob travel (it's a 10k pot). I've attached a sample of my filter in action.

My version of the circuit has 0.01uF poly caps and a 272K resistor around the op-amp, running on a 9V power supply. There is a slight but audible difference between the poly caps and cheap ceramic ones.

The choice of op-amp makes a significant difference in audio quality.

LM324 (quad): Nice filter cutoff throughout the filter range, the resonance knob will self-oscillate if turned past about 90%, below that the resonance is a bit thin. There's a bit of unfiltered "sparkle" in the high end. Will operate at 5V, but I hear significant distortion/clipping.

LM358 (dual): Sounds the same as the LM324 (not really a surprise). Self-oscillates when resonance is turned up full.

TL074CN (quad): Resonance knob doesn't self oscillate when cranked, sounds similar to the audio sample I posted. HOWEVER, the cutoff has an audible bump as it's turned all the way down. This was a showstopper for me.

TL071CP: The winner. Similar resonance response to the 074, but the cutoff is much smoother throughout the full range. Not sure why there's such an audible difference between the 071 and the 074, given the similar chip specs.

I've breadboarded a little voltage doubler circuit that scales 0-5V cv from my microcontroller's DAC to 0-9V to allow full range control of the filter cutoff.Last edited by reflex on Fri May 10, 2013 12:48 pm; edited 1 time in total

TL074CN (quad): Resonance knob doesn't self oscillate when cranked, sounds similar to the audio sample I posted. HOWEVER, the cutoff has an audible bump as it's turned all the way down. This was a showstopper for me.

That is VERY interesting.

I have a Thomas Henry VCF from AS21C which is supposed to self-oscillate, but does not. Tried a bunch of ideas from Dave Dixon on improvements to the circuit, still does not self-oscillate.

Guess which Op Amps I used?

I wonder whether there is even variation across batches, since the original specs TL074's....

Well that's pretty much exactly what I'm getting at. I hadn't tried, but I plan to. Either 324's or any of a number of other quad op amps with compatible pinouts (LF444 comes to mind, though I only have 1 left).

Well, both the LM324 and the LF444 made it quite a bit more aggressive, but the high end of the pot (past about 3 o'clock) breaks up and sounds like crap, and it still doesn't really self-oscillate in any manner that tracks the frequency knob.

Of course along the way, one of the sockets on my cheap leaf-spring socket gave up the ghost, and I had to troubleshoot that too. Nothing like stuff you built before you realized particular pitfalls .

But in the end the TL074, while not as aggressive, has the best range with the pot, and since none of the options self oscillate, I've gone back to it.

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